SCHEDULE-DEPENDENT SYNERGISM BETWEEN RALTITREXED AND IRINOTECAN IN HUMAN COLON-CANCER CELLS IN-VITRO

The quinazoline folate analogue raltitrexed (ZD11694; Tomudex) and the camptothecin derivative irinotecan are two new agents showing clinica l activity against colorectal cancer, To identify the optimal conditio ns to achieve synergistic cytotoxicity before the clinical development of their combination, we explored the interactions between ZD1694 and the active metabolite of irinotecan, 7-ethyl-10-hydroxycamptothecin ( SN-38), in vitro, Cytotoxicity was evaluated with a clonogenic assay u sing the human colon cancer cell line HCT-8. Different schedules of ad ministration and different dose ratios of the two agents were compared and evaluated for synergism, additivity, or antagonism with a quantit ative method based on the median-effect principle of Chou and Talalay (T. C. Chou and P. Talalay, Adv, Enzyme Regul., 22: 27-55, 1984), Sequ ential short-term (1 and 4-h) exposures to SN-38 followed by ZD1694 re sulted in synergistic cytotoxicity at broad dose-effect ranges, At a h igh level of cell kill, the synergism was greater when either equiacti ve doses of the two agents or higher relative doses of ZD'1694 were us ed. A 24-h interval between exposure to SN-38 and ZD1694 significantly enhanced the magnitude of the synergy (P = 0.001). The opposite seque nce or simultaneous exposures produced significantly less potentiation or nearly additive interactions (P = 0.0006 and P < 0.0001), The syne rgism was completely lost under conditions of more prolonged drug expo sure (24-h continuous exposure). In conclusion, in this in vitro model of human colon cancer, ZD1694 and SN-38 produced synergistic cytotoxi city, Our findings support the clinical use of this combination and pr ovide a rationale for a clinical trial using sequential shortterm expo sures to equiactive doses of SN-38 and ZD1694 administered sequentiall y with a 24-h interval.